It is remarkable that semiconductor integrated circuit devices have been highly integrated and therewith developed to be finer. For improving process yield of device production, there is strong requirement for enlargement in size and higher quality of a wafer used as a substrate. Items which relate to crystal quality such as oxygen concentration of a substrate and heavy metal impurity affect property of a semiconductor integrated circuit device (See, Ultra Clean Technology Vol. 5 NO 5/6 “heavy-metal contamination and oxide-film defects of a silicon wafer”), particularly it has been reported that dielectric breakdown strength of a gate oxide film of MOS is degraded by heavy-metal contamination such as Fe, and the like. Moreover, in the case that a silicon single crystal is contaminated by heavy metal, this has a great influence on lifetime of minority carriers and has possibilities of causing problems in property of the semiconductor integrated circuit device.
Moreover, in particular, as an important point for improving process yield in device production in recent years, improvement of device yield in a peripheral part of a wafer has become a problem. Therefore, it has become important to reduce contamination of heavy metal such as Fe in the peripheral part of a wafer. As a cause of the heavy-metal contamination of a single crystal, there is impurity mixed in melt. And, it was recently found that Fe (iron) released from a gas flow-guide cylinder and such adheres to a single crystal during the pulling.
In CZ method, in particular, in the case that a silicon single crystal having a large diameter of 200 mm or more is grown, there is frequently used an apparatus in which a gas flow-guide cylinder so as to surround the single crystal pulled from a raw material melt is disposed. The gas flow-guide cylinder is also important for straightening flow of an inert gas provided in a chamber during the growth and efficiently exhausting out of the furnace a silicon oxide that evaporates from the melt. As a general gas flow-guide cylinder, a carbon material such as a graphite member is used and disposed to be close to a crystal by a distance in the range of 10 mm to 200 mm from the crystal or further by a distance of 10 to 100 mm. Moreover, as material of the gas flow-guide cylinder, high melting point metal such as tungsten or molybdenum is occasionally used. Furthermore, in the case that an appropriate cooling medium is used, stainless or copper can be used as material of the gas flow-guide cylinder.
However, if a heavy-metal component such as Fe is released from the gas flow-guide cylinder, it adheres to a crystal surface during the growth, and Fe is diffused toward the crystal center from the crystal periphery along with the later growth in a cooling process from an ultra high temperature during the crystal growth to a room temperature, particularly metal contamination is occasionally caused in the peripheral part of the crystal.
As measures for heavy-metal contamination caused by the gas flow-guide cylinder as described above, it has been proposed that a surface of the gas flow-guide cylinder is coated by a high-purity coating film of pyrolytic graphite that a Fe concentration is suppressed to be very low, or the like (see, International Publication WO 01-81661). By coating a surface of a gas flow-guide cylinder as described above, release of a Fe component from the gas flow-guide cylinder can be suppressed and Fe concentration even in a peripheral part of a grown single crystal can be suppressed to be low.
On the other hand, as devices become highly integrated in recent years, it is also demanded to reduce grown-in defects such as FPD, LSTD, and COP in a wafer. Grown-in defects are defects caused by single crystal growth which are induced in a crystal during growth when a silicon single crystal is grown by CZ method.
Hereinafter, there will be described relation between a pulling rate when a silicon single crystal is grown by CZ method and defects in the silicon single crystal to be grown. It is known that in the case that a growth rate V is changed from a high speed to a low speed in the direction of the crystal axis by a CZ pulling apparatus, a cross-section in an axial direction of the single crystal can be obtained as a defect distribution view as shown in FIG. 8.
V region in FIG. 8 is a region having a number of Vacancies, i.e. concave parts generated by silicon atom shortage, such as holes. I region is a region having a number of dislocations or a number of bodies of excess silicon atoms which are generated by existence of Interstitial-Si, which is an excess silicon atom. Neutral region (N region) having no or little shortage or excess of atoms exists between the V region and the I region. Moreover, defects, which are referred to as OSF (Oxidation Induced Stacking Fault) in the vicinity of a boundary of the V region, are distributed in a ring shape (OSF ring) when viewed in a cross-section in a vertical direction to crystal growth axis (in a surface of the wafer).
In the case that the growth rate is relatively high, grown-in defects such as FPD, LSTD, and COP originated from voids that vacancy-type point defects aggregate exist at high density in the entire region in the radial direction of a single crystal and the region that these defects exist becomes V region. As the growth rate becomes lower, OSF ring is generated from the crystal periphery and N region is generated in the outside (the lower rate side) of the ring. Furthermore, if the growth rate is low, the OSF ring shrinks to the center of the wafer and disappears and the entire plane thereof becomes N region. If the growth rate is further lower, L/D (Large Dislocation: general designation of “interstitial dislocation loop”, such as LSEPD or LFPD) defects (large dislocation clusters), which are thought to be originated from dislocation loops that interstitial silicones aggregate, exist at low density and the region that these defects exist becomes I region (occasionally referred to as L/D region).
N region outside the OSF region between the V region and the I region becomes a region having low defect density, in which there exist neither FPD, LSTD, and COP that are originated from vacancies, nor LSEPD and LFPD that are originated from interstitial silicones. In recent days, it has been found that if the N region is further categorized, as shown in FIG. 8, there are Nv region (a region where vacancies exist predominately) next to the outside of the OSF ring and Ni region (a region where interstitial silicones exist predominately) next to I region. When thermal oxidation treatment is performed, in the Nv region, amount of precipitated oxygen is rich, and in the Ni region, amount of precipitated oxygen is little.
In recent years, in CZ method, by setting growth rate of a crystal to be low or by setting a structure inside a furnace of the CZ pulling apparatus for gradual cooling of the crystal, it has become possible to produce a silicon crystal with low defect density in the entire crystal.
For example, there is proposed a method that by controlling thermal history during crystal growth, point defects are reduced (See, Japanese Patent Application Laid-open (kokai) No. 9-202684, and No. 7-41383). Moreover, by controlling V/G, which is a ratio of a pulling rate (V) to an axial temperature gradient (G) at a solid-liquid interface in a crystal, it has become possible to produce a crystal that N region is expanded in the horizontal entire plane (the entire plane of a wafer) (See, Japanese Patent Application Laid-open (kokai) No. 8-330316 and No. 11-147786).
In the case that a crystal with low defect density is grown as described above, for improving process yield of device production, it is also important to reduce contamination of heavy metal such as Fe.
However, when a silicon single crystal having low defect density is produced, even if there is used a gas flow-guide cylinder coated with a coating film that Fe concentration is extremely low as described above, Fe concentration in a peripheral part of the crystal cannot be suppressed to be sufficiently low, and it has been difficult to make 1×1010 atoms/cm3 or less like demands in recent years. Therefore, there has been a problem that process yield becomes low in the production of semiconductor devices after that.
Moreover, as a method for producing a silicon single crystal having low defect density with Fe contamination suppressed, there has been proposed a method that raw material is cleaned with fluoric acid and such, and a single crystal ingot is pulled by a constant rate (a solidifying rate) from a melting raw material. Furthermore, after chunked or grained, this is cleaned and melted again, and thereafter a silicon single crystal is grown by controlling V/G (See, Japanese Patent Application Laid-open (kokai) No. 2000-327485). According to such a method, it is supposed that there can be grown a silicon single crystal having no grown-in defects which Fe concentration is reduced to be 2×109 atoms/cm3 or less.
However, in a method for growing a silicon single crystal wherein Fe concentration is reduced by repeating the cleaning of raw material, the melting, and the pulling as described above, cost significantly increases because pulling is performed twice or more. Even if such a method is used, there is a problem that there cannot be avoided Fe contamination caused by a gas flow-guide cylinder during growth.